UV Unwrap: Unwrap Mesh to fixed pixel per unit
David's comment was the answer I was looking for.
TexTools is perfect for that. While in UV Editing mode it allows you to calculate the actual Texel-per-Unit-Ratio. You can also set a specific texel density manually which scales the UV islands to that specific size.
It also offers many more options for UV editing.
A smaller version only to change the texel density ist the Texel Density Checker on Gumroad.
Both tools work together without interfering each other.